This invention relates to nuclear reactors and more particularly to apparatus for fastening fuel pins to the attachment rails which support the fuel elements of a reactor.
The upright parallel fuel pins which contain the fissionable material at the core of a reactor are commonly supported through bottom end members which are engaged on transverse attachment rails. More particularly, the attachment rails are typically of circular cross section and extend along the upper edges of upstanding support plates of less thickness. The fuel pin lower end member has an upwardly extending slot with a width similar to the thickness of the upstanding plate and which terminates at a transverse passage of circular cross section through which the rail extends. The lower ends of such fuel pins are thus locked to the rails and the pins cannot be removed by vertical forces. An annular housing encircles the fuel pin bundle to prevent lateral displacement of the pins.
It is not possible to disengage a particular individual fuel pin of the kind described above from the rail and then remove it from the core except through an exceedingly complex, time consuming and costly sequence of operations. Removal of such a fuel pin requires that it be moved sidewardly off the end of the associated rail. If it is one of the interior pins, this cannot be done without first removing others of the pins that are attached to the same rail.
Space constrictions and the typically high level of radioactivity do not permit such operations to be performed at the core region of most reactors. As a practical matter, retrieval of a particular individual fuel pin of the above described type from a reactor requires removal of the fuel pin assembly as a whole after which the above described operations must still be performed using, at least in most cases, complex remotely controlled tools.
The problems discussed above in connection with removal of a fuel pin from the reactor core are, for essentially similar reasons, also encountered where an individual fuel pin is to be installed into the reactor core.
It is highly advantageous, at least in some reactor operations, if one or more specific fuel pins can be more simply and conveniently retrieved after a period of reactor operation and if the same pin or a replacement can be reinstalled without an extensive disassembly of the reactor structure. Among other advantages, this capability greatly facilitates analysis of the chemical and physical changes which occur in fuel pins in the course of reactor operation. A retrievable fuel pin cannot be locked to the attachment rail in the manner described above. To enable retrievability, a typical prior practice has been to provide a slotted lower end member which fits onto the rail but which offers no significant resistance to upward withdrawal of the fuel pin. A stop plate is situated above the fuel pin assembly. To hold the retrievable pin down, it has been made more lengthy than the other fuel pins so that the top of the retrievable pin protrudes above the level of the tops of the other pins to almost contact the stop plate. Among other disadvantages, this arrangement does not provide a desirable amount of tolerance for axial growth of the retrievable fuel pin. It would be preferable to provide for fastening of a retrievable fuel pin at the lower end rather than at the top.